1. Field of the Invention
The invention relates to a CNC controlled buttonhole sewing machine, comprising a needle, which is drivable via an arm shaft; an x-y table, which is movable by an x drive in an x direction and by a y drive in a y direction, and which supports a first and a second work piece clamp, the first and second work piece clamps being mounted for displacement relative to each other, and which comprises a sliding drive for displacement of the work piece clamps from an initial position of spread by a length of spread into a final position of spread; and a control unit for control of the drives for displacement of the x-y table prior to the start of production of a buttonhole seam.
2. Background Art
The JUKI MEB-3200 Instruction Manual No. 02 29343316 leaflet describes a CNC controlled buttonhole sewing machine of the generic type. It comprises an x-y table which is displaceable on the sewing plane by two stepper motors as x and y drives. Two work piece clamps are disposed on the x-y table, holding a work piece thereon. By means of a pneumatic cylinder, they are displaceable in the x direction on the sewing plane, which is formed by the x-y plane, in mirror symmetry to a center plane. Both pneumatic cylinders are operable by means of a control command that is stored in the control unit. Provided on each side of the x-y table are setscrews, enabling the length of spread of each work piece clamp to be set from a non-modifiable inner initial position of spread to a final position of spread.
Shifting the work piece clamps enables the tightly clamped work piece to be stretched and spread into a tautened plain position. It is possible in this way to produce high quality buttonholes. Stretching the work piece also creates sufficient room for the needle, in case the buttonhole is first cut and then sewn i.e., with the sewing machine working in a so-called pre-conditioning mode. The displacement of each work piece clamp is in the range of 1.0 mm so that the clamps can be moved apart by a length of spread of 2.0 mm. The length of spread depends on various sewing parameters such as the structure of the work piece i.e., material thickness and the like, the kind of threads used for sewing, thread tightening, needle size, possible use of a gimp thread and further parameters.
Drawbacks of the familiar sewing machine reside in that the initial position of spread and the length of spread i.e., the final position of spread, can only be obtained by time-consuming adjustment with the aid of implements such as a screw driver and gauge. This job requires a learned mechanic. The numerous components needed for adjustment are susceptible to wear. It is difficult to retrieve and reproduce set-ups that have been determined empirically. Every modification in buttonhole width requires renewed setting; otherwise it is not possible to clamp the work piece in proximity to the buttonhole seam that is to be produced.
German patent 457 750 describes a spreading mechanism for the work piece clamping arrangement of buttonhole sewing machines which includes work piece clamps that are displaceable, by a mechanism, in symmetry to a center plane. Provision is made for a lever mechanism with a manually adjustable stop, the setting of which modifying the length of spread. This known mechanism also has the above-mentioned drawbacks.
U.S. Pat. No. 4,312,283 teaches an automatic sewing machine, in which a CNC control unit generates coordinates of motion of a sewing machine relative to a work piece holder of adjustable size. For preventing the needle from colliding with the work piece holder, information on the respective size set in the work piece holder is fed to the control unit for the control unit automatically to make a corresponding correction in a sewing program.
It is an object of the invention to further develop a CNC controlled buttonhole sewing machine of the generic type so that the range of spread can be set by an operator to comply with respective sewing requirements at a minimum expenditure of time and without any need of aids.
According to the invention this object is attained by the features which are characterized in that the sliding drive at least of the first work piece clamp is an electric positioning motor; and in that the control unit includes a data storage with data stored therein for displacement of the at least one first work piece clamp by a given length of spread. The solution according to the invention ensures that the length of spread can be given very conveniently by control unit input so that the ranges in length of spread that correspond to the respective sewing conditions are easily adjustable. This includes work piece quality, buttonhole seam parameters, ranges of tightening of the threads sewn. This will increase the quality of a buttonhole by very precise adjustment of the final position of spread after displacement, by the length of spread, of the work piece clamps from the initial position of spread into the final position of spread. Initial position of spread means the distance of the work piece clamps in their close side-by side position of rest. Final position of spread means a position of the work piece clamps in which they are moved out of the initial position by the length of spread. These adjustment and input jobs can be managed by the operator which means great convenience of handling. Adjustment is feasible without the need of special aids. Machine down-times are kept low because very little time is consumed by the adjustment jobs. The operating mechanic is relieved. The entered control data constitute a basis for further processing of the set-ups in a display and/or for calling stored sewing programs. Manufacturing high quality buttonholes becomes reproducible.
In keeping with a further development, data are stored in the data storage, for displacement at least of the first work piece clamp in relation to the second work piece clamp into an initial position of spread. This ensures that also the initial position of spread can be suited to the envisaged shape and size of a buttonhole seam, enabling the work piece to be clamped in proximity to the seam that is to be sewn. This results in especially reliable stitch forming and excellent buttonhole quality.
In keeping with the especially advantageous embodiment, only a single work piece clamp must be movable on the x-y table, a compensating motion being produced by the possibility of displacement of the x-y table. This reduces the number of components required, leading to more reliable operation and a lower manufacturing cost.
High simplicity of design is accomplished in that the first and the second work piece clamp are displaceable one in relation to the other in an opposite direction, and in that the first and second work piece clamps are displaceable by only a single sliding drive.
In keeping with an advantageous embodiment, no forces are transmitted to the guides of the work piece clamp on the x-y table at least by the displaceable work piece clamp. No frictional forces that might oppose the displacement of the work piece clamp for spreading are occasioned in positioning the work piece clamp. The forces that occasion when a work piece is clamped are kept within the work piece clamp itself. This works in favor of the rapidity and accuracy of the spreading job. Lightweight construction of the entire spreading arrangement is possible, saving material consumption.